The most fuel-efficient U.S. domestic passenger airline was 26% better than the least fuel-efficient one in 2010, according to a report released today by the International Council on Clean Transportation (ICCT), a nonprofit research organization based in Washington, D.C. The study is the first to quantify that performance gap using publicly available data and accounting for differences in business operations across airlines.

Alaska Airlines led all fifteen mainline carriers operating in the U.S. in 2010, followed closely by Spirit Airlines and Hawaiian Airlines. Allegiant Air was the least fuel efficient carrier, trailing American Airlines and AirTran Airways.

The ICCT analysis is based on fuel-consumption data reported annually by airlines to the U.S. Bureau of Transportation Statistics (BTS Form 41). It employs a new methodology, developed by a team of researchers at FAA’s National Center of Excellence for Aviation Operations Research (NEXTOR) at the University of California, Berkeley, to evaluate an airline’s fuel efficiency relative to both the mobility (straight-line passenger miles between origin and destination) and access (airports served and/or flight frequency) it provides. This approach permits a precise distinction between fuel burned to provide a given level of service and fuel burned as a result of inefficiencies, such as the use of older technology, circuitous routing, or taxiing with two engines instead of one.

“The frontier-analysis approach developed for this study applies a well-recognized method of productivity analysis in a novel manner that reflects the unique features of this problem,” said UC Berkeley’s Mark Hansen one of the researchers whose work provided a foundation for the report. “To do a fair comparison you have to find a way to adjust for varying business models, network structures, and scale, and to properly account for regional carriers. This methodology does all that.”

The ICCT plans to update the report annually, with numbers for 2011 slated for release later this year. “Getting the methodology right and establishing this baseline was the hardest part,” said Dan Rutherford, one of the report’s coauthors. “Because we rely strictly on publicly reported data, the updates will be pretty straightforward.”

The report attributes roughly a third of the variation in fuel efficiency to the level of investment in technologies such as winglets and high-bypass ratio engines, which is related to aircraft age. Factors such as seating density and load factor, ground operations practices like single-engine taxiing, and how well an airline matches aircraft capability with operational mission account for the remainder. Mazyar Zeinali, another of the report’s coauthors, noted that the International Civil Aviation Organization (ICAO), which sets technical standards for aircraft safety and emissions, could address both technical and operational inefficiencies by establishing an aircraft CO2 emission standard and throwing its weight behind some type of market-based measure to impose a price on carbon emissions. “Fuel costs are a big concern for airlines now, but these numbers raise real questions about the conventional wisdom that fuel costs by themselves act as a sufficient incentive to drive efficiency in this sector.” The ICAO General Assembly will convene in Montreal beginning on September 24, 2013.

Fuel accounted for approximately 35% of operating expenses for the fifteen airlines included in the study. But, counterintuitively, the study found that fuel efficiency was not well correlated with profitability. Of the six most-efficient airlines, two, Continental (#4) and Frontier (#6) operated at a net loss over the three-year period on their domestic operations. Of the two least-efficient airlines, one, Allegiant, was the most profitable carrier over the three-year period leading up to 2010, while the other, American, entered bankruptcy soon after. But the apparent lack of correlation may be misleading: in addition to American Airlines, three others of the five worst-performing carriers—Delta, US Airways, AirTran Airways—has since been involved in mergers.

In addition to overall airline fuel efficiency, the study analyzed the ten most important city-to-city routes in the continental U.S. On those routes the estimated difference between the most- and least-efficient airlines was even greater than overall, ranging from 9% (San Francisco–San Diego) to 87% (New York–Chicago). But airline performance on specific routes did not track overall efficiency closely: in several cases, the most fuel-efficient airline flying between cities was below average overall. “On any given route, something may give an airline an advantage,” said Rutherford. “AirTran and Delta are the most fuel-efficient carriers between Atlanta and New York, because Atlanta is a major hub for both. But overall, they’re both relatively inefficient.”

The International Council on Clean Transportation is an independent nonprofit organization founded to provide first-rate, unbiased research and technical and scientific analysis to environmental regulators. Its mission is to improve the environmental performance and energy efficiency of road, marine, and air transportation, in order to benefit public health and mitigate climate change.